Abstract: A system for the indirect detection of agglutination in an agglutination assay using optical density measurement and a reaction vessel. Light emitted by light source is transmitted through reaction vessel containing components of an agglutination reaction that is detected by light detector. As particles in the reaction vessel settle in the absence of agglutination they are concentrated in a reaction vessel lower volume and increase the optical density of the light path through the reaction vessel. Light detector generates an output signal related to said optical density. Said output signal is read and interpreted by an instrument to detect a presence or absence of agglutination. Other embodiments are described and shown.